Any of the long boards laid over joists to make a floor.
There was a rat that lived under our duplex. […] It went in and out through a broken floorboard in the porch, and into our kitchens, but we didn’t believe in killing it.1976, Vance Bourjaily, “All Quiet on the Middlewestern Front”, in Now Playing at Canterbury, New York, N.Y.: The Dial Press, →ISBN, page 364:
The officers said they found two rifles in the car, one on the front floorboard of the vehicle and another on the rear floorboard.1999, Ernesto B. Vigil, The Crusade for Justice: Chicano Militancy and the Government's War on Dissent, page 212:
To sink the gas pedal into the floorboard of the car, in order to bring the car to the highest possible speed.
I floorboarded the gas pedal and cut over sharply to the right, threatening the Cadillac's fenders and its driver's life.1953, Ross Macdonald, Gone Girl: